remove MetricType class

[donbourne]

The ability to register an arbitrary subtype of Metric has been removed from the MetricRegistry class (in the 5.0 release). As such, the only way to create metrics is from the Counted / Gauge / Timed annotations or the MetricRegistry counter / gauge / histogram / timer methods. As a result, it should always be obvious to the implementation what the metric type is, and we can eliminate the MetricType class. A benefit of doing this is that we would no longer give users of the API the ability to get into trouble by trying to create a Metadata object with a metric type (say COUNTER), and then try to use that to create a different type (say TIMER). It would also help tidy up the API to not have this unnecessary class.

• remove MetricType class
• remove MetricType from Metadata / MetadataBuilder / DefaultMetadata

[donbourne]

fixed by #740